What is wrong then the car jumps or feels as though it is skipping gears? on 2005 Pontiac Grand Prix

There are not any codes.

Engine misfire, need a tune up!
1 more answer
From the frequently reports problems page:

20 reports of Erratic Shifting of the Automatic Transmission

The transmission pressure control solenoid may fail causing erratic shifting. Our technicians tell us that partial dis-assembly of the transmission is necessary to replace a failed pressure control solenoid.